Natural and synthetic zeolitic materials have been demonstrated to have catalytic properties for various types of hydrocarbon conversion. Certain zeolitic materials are ordered porous crystalline aluminosilicates having a definite crystalline structure as determined by X-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties.
Such molecular sieves include a wide variety of positive ion-containing crystalline aluminosilicates. These aluminosilicates can be described as a rigid three-dimensional framework of SiO.sub.4 and AlO.sub.4 in which the tetrahedra are crosslinked by the sharing of oxygen atoms whereby the ratio of the total aluminum and silicon atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example an alkali metal or an alkaline earth metal cation. This can be expressed in a manner such that the ratio of aluminum to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged either entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given aluminosilicate by suitable selection of the cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great variety of synthetic aluminosilicates. These aluminosilicates have come to be designated by letter or other convenient symbols, as illustrated by zeolite A (U.S. Pat. No. 2,882,243), zeolite X (U.S. Pat. No. 2,882,244), zeolite Y (U.S. Pat. No. 3,130,007), zeolite ZK-5 (U.S. Pat. No. 3,247,195), zeolite ZK-4 (U.S. Pat. No. 3,314,752), zeolite ZSM-5 (U.S. Pat. No. 3,702,886), zeolite ZSM-11 (U.S. Pat. No. 3,709,979), zeolite ZSM-12 (U.S. Pat. No. 3,832,449), zeolite ZSM-20 (U.S. Pat. No. 3,972,983), ZSM-35 (U.S. Pat. No. 4,016,245), zeolites ZSM-21 and ZSM-38 (U.S. Pat. No. 4,046,859), zeolite ZSM-23 (U.S. Pat. No. 4,076,842), and the like.
The SiO.sub.2 /Al.sub.2 O.sub.3 ratio of a given zeolite is often variable For example, zeolite X can be synthesized with SiO.sub.3 /Al.sub.2 O.sub.3 ratios of from 2 to 3; zeolite Y, from 3 to about 6. In some zeolites, the upper limit of SiO.sub.2 /Al.sub.2 O.sub.3 ratio is essentially unlimited. ZSM-5 is one such example in which the SiO.sub.2 /Al.sub.2 O.sub.3 ratio can vary from about 5 up to a ratio which approaches infinity. U.S. Pat. No. 3,941,871 (now U.S. Pat. No. Re. 29,948) discloses a porous crystalline silicate made from a reaction mixture containing no deliberately added alumina in the formulation and exhibiting an X-ray diffraction pattern characteristic of ZSM-5 type zeolites. U.S. Pat. Nos. 4,061,724; 4,073,865; and 4,104,294 describe crystalline silicates or organosilicates of varying alumina and metal content.
The prior art also discloses methods for incorporating into zeolitic materials strong hydrogenation-dehydrogenation metal components as illustrated by metals such as molybdenum, chromium and vanadium, and Group VIII metals such as cobalt, nickel and palladium.
U.S 3,201,356 describes a method for activating a crystalline zeolitic molecular sieve catalyst composited with a noble metal component which involves dehydrating said catalyst to a water content of less than 1.8 weight percent at a temperature below 320.degree. F., and thereafter heating the catalyst in the presence of hydrogen at a temperature of about 320.degree. F.
U.S. Pat. No. 3,700,585 in columns 7-8 reviews the typical ion exchange techniques employed for introducing metal cations into zeolite structures, such as the techniques described in U.S. Pat. Nos. 3,140,249; 3,140,251; and 3,140,253. As a general procedure, a particular zeolite is contacted with a salt solution of the desired replacing cation. The zeolite is then preferably washed with water, dried at 65.degree.-315.degree. C., and calcined in inert atmosphere at 260.degree.-815.degree. C.
U.S. Pat. No. 3,956,104 describes a hydrocracking catalyst which is prepared by a series of steps which include (1) admixing ammonium hydroxide and aluminum sulfate in an aqueous medium to form a soluble aluminum sulfate partial hydrolysis product; (2) admixing a crystalline aluminosilicate zeolite with the partial hydrolysis product, effecting complete hydrolysis of the aluminum sulfate, and ageing the resulting mixture for about two hours; (3) separating and washing the solids; and (4) impregnating the solids with calculated quantities of Group VIB and Group VIII metal components, and calcining the resulting composite.
U.S. Pat. No. 4,148,713 describes the preparation of ZSM-5 type of crystalline aluminosilicate zeolites which have particles coated with an aluminum-free outer shell of silica. Optionally the zeolites can contain metal cations of hydrogenation components such as Group VI and Group VIII metals.
U.S. Pat. No. 4,174,272 describes zeolite catalysts containing platinum group metals, which are employed in non-hydrogenative endothermic catalytic cracking of hydrocarbons in a system wherein the endothermic heat required for cracking is supplied by the catalyst as the heat transfer medium.
The prior art crystalline aluminosilicate zeolites of the type described above generally exhibit acid activity, in their hydrogen form, e.g., they have a relatively low silica/alumina ratio. Acidity and ion-exchange capacity are related to the aluminum content of a zeolite. A high silica/alumina zeolite exhibits relatively low acid activity in the hydrogen form.
The shape-selective properties of the prior art zeolites are adapted for acid-catalyzed reactions such as cracking of hydrocarbons. The said prior art zeolites are not particularly effective for shape-selective metal-catalyzed reactions such as shape-selective dehydrogenation and dehydrocyclization, e.g., for the conversion of n-paraffins to aromatic products in the presence of cycloparaffins.
Accordingly, it is an object of this invention to provide a method for preparing a crystalline zeolite catalyst composition which contains a shape-selective metal function, and which exhibits a reduced acid activity.
It is another object of this invention to provide a base-exchanged crystalline zeolite composition which exhibits an X-ray diffraction pattern characteristic of a ZSM-5 or ZSM-11 type of zeolite structure, which has a high capacity for ion-exchanging metal cations, and which contains a shape-selective metal function and exhibits little or no acid-catalyzed reactivity.
It is a further object of this invention to provide a process for the production of aromatic hydrocarbons from paraffinic feedstock in the presence of a base-exchanged crystalline zeolite catalyst containing a shape-selective metal function, wherein the catalyst has an ultra-high silica/alumina ratio and exhibits an X-ray diffraction pattern characteristic of a ZSM-5 or ZSM-11 type of zeolite structure.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.